Wireless Personal Area Network (hereinafter referred to as ‘WPAN’) technology is one of the core technologies for ubiquitous networks in which persons, computers and objects are connected to each other through a single structure. A representative example of the application of WPAN technology is a sensor network, which functions to combine an existing sensor network environment with an actual physical environment.
A sensor network occupies an important role in realizing the ubiquitous generation. In particular, when ubiquitous networking is implemented in the home, it will provide a large incentive to construct national-level infrastructure, and thus the construction of a ubiquitous networking environment in the home is very significant. Further, home networking technology, which is one of the ubiquitous networking technologies, has emerged as a core technology for overcoming the serious situation in which existing markets for electric home appliances are rather depressed, in the ubiquitous generation, and a sensor network is central to such home networking technology.
It is predicted that a sensor network, expected to be composed of several hundreds or thousands of small sensor modules, will be applied to various fields, such as remote monitoring in intelligent home networks, automatic manufacturing process control, the administration of warehouse and physical distribution, remote patient monitoring in hospitals, and security systems for break-in detection.
Meanwhile, a WPAN can be used to implement such technology, and is characterized in that it has advantages, such as a transmission range of less than 10 m, low power consumption, and a size small enough to be mounted in a sensor or the like. Of such WPAN technologies, technology that is currently attracting attention includes ZigBee, which is low-speed and low-power WPAN technology. However, current ZigBee technology is limitedly applicable to networks and suffers from the instability of networks.
FIG. 1 is a diagram showing a transmission interval between beacons transmitted by a single node in ZigBee. A single node has a period during which data is transmitted or received after transmitting a beacon, and then has a sleep period in order to reduce power consumption. The intervals at this time are regularly designated and are equally applied to a next beacon transmission period, a next data transmission/reception period, and a next sleep period. That is, the beacon transmission period, the data transmission/reception period, and the sleep period are repeated at regular intervals.
During the sleep period, the transmission of data is possible, but the reception of data is impossible. The transmission of a beacon is required in order to connect a node below a reference node, so that the last node present in the configuration of a network does not require the transmission of a beacon after a certain period of time has elapsed.
FIG. 2 is a diagram showing the status of the transmission of beacons between two nodes occurring at the time of transmitting beacons between a first node and a second node. The first node transmits a beacon to the second node and transmits a time slot, ranging from the transmission of the beacon to the transmission of the next beacon, to the second node. The second node, having received the time slot, transmits its own beacon in the start section of the period, which is not used by the first node, in order to avoid collisions with the beacon transmitted by the first node.
FIG. 3 is a diagram showing the inefficiency of data transmission in a conventional method of setting a beacon slot, in which the inefficiency of data transmission, occurring as nodes have a large depth when communication is performed by configuring beacon transmission time slots using the method of FIG. 2, is shown.
In network configuration in which the depth of nodes is 5, as shown in FIG. 3, when a fifth node attempts to transmit data to a first node, the data must be transmitted during the period in which a fourth node can receive data transmitted from the fifth node.
However, since the period in which the fourth node can receive data has already elapsed, the fifth node must wait for the next period in which the fourth node can receive data. As a result, since the time taken to transmit data up to the first node is increased by one cycle in this way, there is a problem in that the total transmission time increases.
FIG. 4 is a network configuration diagram showing a conventional method of setting a beacon slot, in which a wireless network environment configured using the methods of FIGS. 1 and 2 is shown.
In the drawing, the WPAN environment is configured such that a coordinator has first and second nodes in its own communication range, and the first node and the second node have third and fourth nodes and fifth and sixth nodes in their own communication ranges, respectively.
FIG. 5 is a diagram showing an example of the setting of a beacon slot in the network environment of FIG. 4.
As shown in the drawing, in the case where the beacon transmission interval is configured using the method of FIG. 2 in the network environment of FIG. 4, when the first and second nodes perceive the beacon transmitted from the coordinator, the first and second nodes cannot recognize each other in their own communication ranges, and thus they transmit their beacons after the same interval has elapsed from the time point at which the coordinator transmits the beacon.
In this case, as described above, since the first and second nodes are not located in their communication ranges, there is no problem in transmitting beacons so as to set up connection to third, fourth, fifth and sixth nodes and perform data transmission even if the beacon transmission time slots of the first and second nodes are identical to each other. However, if another node appears and sets up a new connection, a collision between beacons may occur. This phenomenon is described in detail with reference to FIGS. 6 and 7.
FIG. 6 is a diagram showing network configuration formed when a new node joins the network of FIG. 4, and FIG. 7 is a diagram showing an example of the setting of a beacon slot in the network environment of FIG. 6.
As shown in FIG. 6, a seventh node, appearing and newly joining the network, enters the communication range of a coordinator, and the communication range of the seventh node includes first, second, fourth and fifth nodes.
In this case, when beacon transmission time slots are configured using the methods of FIGS. 2 and 5, the seventh node receives a beacon from the coordinator, and calculates my beacon transmission time slot. Therefore, the beacon transmission time slot of the seventh node is set as the same time as that of the first and second nodes, as shown in FIG. 7.
When the network is configured in this way, there is no problem in the communication between the coordinator and the first, second and seventh nodes because the first, second and seventh nodes need to receive only the beacon from the coordinator. However, when the seventh node is generated after the network of FIG. 4 has been configured, some problem may occur.
That is, as shown in FIG. 6, the communication range of the seventh node includes fourth and fifth nodes, and the beacon transmission time slot of the seventh node is identical to that of the first and second nodes. Accordingly, if the first, second and seventh nodes simultaneously transmit beacons, a beacon collision occurs in the third node because the first and seventh nodes simultaneously transmit the beacons, and occurs in the fourth node because the second and seventh nodes simultaneously transmit the beacons. Due thereto, there are disadvantages in that a previously configured network is broken and is prevented from performing communication, and thus it is impossible to configure a mesh network enabling a more powerful communication network to be formed in wireless communication for which various communication networks can be configured.